Compressed images that are compressed using transform compression methods are common. For example, for still images, the JPEG standard, and for moving images, standards such as motion JPEG, MPEG1, MPEG2, MPEG4, ITU-T H.261, H,263, MPEG4 part 10 (AVC), also known as ITU-H.264, and VC-1 are known and common. In the case of video, each of these compression methods include partitioning an image into blocks, e.g., 8 by 8 blocks, estimating motion, compensating for motion to determine motion compensated residual image blocks, transforming the image blocks, and quantizing the transform coefficients. The quantized transform coefficients are then ordered, e.g., along a zig-zag path on the two-dimensional transform domain, and the ordered series of quantized coefficients entropy encoded, e.g., using the two-dimensional variable length coding method such as originally described in U.S. Pat. No. 4,698,672 to inventor Chen.
There often are situations when image data is provided in already compressed form. For example, in video distribution, MPEG-2 is commonly used. In a video distribution chain, there may be some links that are particularly expensive. For example, there may be one or more satellite links in the distribution chain. Alternatively, in distributing images over cellular telephones, the bandwidth is rather limited. In each such situation, there is incentive to use an efficient compression method at least for the most critical link. One way of increasing the efficiency of distribution includes transcoding from a legacy compression method, e.g., MPEG-2 to newer more efficient methods used, e.g., H264. In some configurations, re-transcoding back to the legacy compressed form, e.g., MPEG-2 may be necessary at the other end of the critical link(s). This however requires full transcoding from the legacy method to a newer more efficient method, and such full transcoding may generate additional quantization errors and is computationally intense, and thus may not always be desirable.